Showing papers in "Transactions of The Faraday Society in 1968"

Symmetries of activated complexes

Abstract: Previous work has indicated that in calculating rates using activated-complex theory one should omit symmetry numbers from the rotational partition functions and multiply by a statistical factor; this factor is the number of equivalent forms of the activated complex that can arise from the reactants. This procedure can lead to error if the activated complex is chosen to have such a high degree of symmetry that it can form more than one form of the reactants or products. It is shown from a consideration of multi-dimensional potential-energy surfaces that a single activated complex cannot exist at the intersection of two valleys; three valleys, for example, will give rise to three separate activated states, each of which represents the lowest pass between a pair of valleys. The implications of this conclusion are considered with reference to several reactions, including that between hydrogen and iodine.

Thermodynamics of polymer solutions. Part 1.—Natural rubber and benzene

Abstract: Activities of rubber-benzene solutions have been determined at 10, 25 and 40°C over the range of concentration, expressed as the segment fraction φ2 of polymer, from 0.05 to 0.945, using high pressure osmometry and an improved vapour sorption method. Results are in general agreement with earlier work of Gee and co-workers, except that they yield lower values for the reduced partial molar excess enthalpy : χ=text-decoration:overlineHE1/RTφ22≅ 0.1 ± 0.1. Excess volumes, also determined at 25°C, are small and positive, amounting to ca. 0.09 % at φ2= 0.5. These results, including the excess volumes, are compared with calculations according to recent theory which takes account of the equation-of-state characteristics of the pure components. Good agreement is established. The small experimental entropy of dilution compared with that calculated from the combinatorial contribution alone is well explained by equation-of-state contributions. The observed increase of the conventional free-energy parameter χ with concentration arises from the latter contribution and is unambiguously predicted by theory.

Statistical thermodynamics of mixtures of molecules of different sizes

Abstract: The central problem in the theory of mixtures is the calculation of the free energy of mixing of molecules of different sizes. An explicit calculation of this free energy is made for a mixture in which all intermolecular potentials are of the form uαα(r)=(σαα/r)n, where σαα is a distance characteristic of the interaction of two molecules of species α, and where σαβ= 1/2(σαα+σββ) when α≠β. This result follows from a solution of Percus-Yevick integral equation for the pair distribution function of a mixture of hard spheres. The form of the free energy provides a criterion by which existing theories can be judged, and it is shown that an approximation of the type originally suggested by van der Waals is superior to approximations based on the concept of random mixing. Molecules that differ only in size mix with a small and negative excess free-energy. The recommended approximation is confirmed by comparison with experiment.

Longitudinal capillary waves. Part 1.—Theory

Abstract: The existence of hitherto unobserved surface waves with a longitudinal character is deduced theoretically and the properties of these waves are predicted. In contrast to capillary waves, the longitudinal waves are governed by the surface elastic modulus rather than by surface tension; they are accompanied by a horizontal rather than by a vertical surface movement and they are connected with a liquid flow of a highly dissipative character causing a rapid damping. Under normal experimental conditions capillary waves and longitudinal waves are superimposed, but the longitudinal wave is usually damped out much more rapidly, except at high elastic modulus.

Kerr effect in inert gases and sulphur hexafluoride

Abstract: The Kerr electro-optic effect has been measured at a series of pressures in helium, neon, argon, krypton, xenon and sulphur hexafluoride. In ideal atomic gases the electric-field induced birefringence is directly proportional to the molecular second hyperpolarizability γ and the gas density ρ. Values for the second hyperpolarizabilities are recorded. Deviations from a linear dependence of the birefringence on ρ have been measured in argon, krypton, xenon and sulphur hexafluoride, and values for the “second Kerr virial coefficients” obtained. These are compared with values calculated from the polarizability anisotropy of an interacting pair of atoms at large separations; agreement is fair, but apparently there are shorter range contributions to the anisotropy.

Pulse radiolysis of aqueous thiocyanate and iodide solutions

Abstract: The transient absorption produced by the pulse radiolysis of CNS– is shown to be due to (CNS)2 formed by the reactions : [graphic omitted] The rate constants of reactions (1), (2) and (3) at 22°C determined from kinetic and equilibrium measurements are 2.8 × 1010, 7.0 × 109 M–1 sec–1 and 3.4 × 104 sec–1, respectively. For iodide ion, which forms I2 in the same way, the equilibrium constant for I2 formation is 1.13 × 105 M–1. The corresponding kinetic constants cannot be determined as accurately as for CNS– but are estimated to be 3.4 × 1010, 7.6 × 109 M–1 sec–1 and 6 × 104 sec–1.

Pulse radiolysis of sulphur compounds. Part 2.—Free radical “repair” by hydrogen transfer from sulphydryl compounds

Abstract: The intense transient absorption spectrum of the radical-anion RSSR– produced in the pulse radiolysis of cysteamine solutions NH2CH2CH2SH, (RSH) by the reaction RS + RS–⇌RSSR– has been used to observe and measure the kinetics of hydrogen transfer reactions from the sulphydryl group to a number of simple organic radicals. This radical repair process is first order in cysteamine concentration and depends upon the dissociation constants of the SH group. In oxygenated solutions containing methanol, oxygen competes with cysteamine for the radical. CH2OH. Rate data are presented and the mechanisms discussed.

Adsorption of sulphur-containing species at the mercury/water interphase

Abstract: The adsorption of SCN– ions and thiourea has been investigated by measuring the capacity of a mercury electrode. Adsorption isotherms for SCN– indicate a large ion-ion interaction which appears to decrease as the bulk solution concentration increases. In contrast, thiourea adsorption can be expressed by a Frumkin isotherm with constant interaction parameter equivalent to a second virial coefficient of 84 A2 molecule–1. The apparent dielectric constant passes through a maximum at a charge on the electrode of about + 6µC cm–2. Thiourea displaces adsorbed nitrate ions from the mercury electrode. Inaccuracies in the drop-time method for determining interfacial tension are discussed and a new method for obtaining an approximate value of the absolute mercury-water interfacial tension is described.

Specific adsorption of chloride ions at the mercury/aqueous solution interface

Abstract: The adsorption of chloride ions on mercury from aqueous solutions of KCl + KF at constant ionic strength has been investigated by measurements of the double-layer capacity as a function of concentration. The adsorption can be represented by a one-constant virial form of isotherm with a non-linear charge dependence of the standard free energy of adsorption in contrast to the adsorption of the same ion from KCl solutions. The hump on the anodic branch of the capacity-potential curve arises partially from the effect of a decrease in the capacity at constant amount adsorbed superimposed on an increasing adsorption capacity, and partially from an inflexion in the charge dependence of the amount adsorbed; this in turn arises from the non-linear charge dependence of the standard free energy of adsorption. The constant charge isotherms for Cl– ion adsorption from KCl and from KCl + KF solutions cannot be superimposed irrespective of the choice of salt activity or mean ionic activity as the concentration variable.

Photolysis of aromatic azides. Part 4.—Lifetimes of aromatic nitrenes and absolute rates of some of their reactions

Abstract: The transients in the flash photolysis of several aromatic azides are identified as triplet nitrenes by their spectra. Their half-lives are determined over a range of concentrations in hexane, ethanol and in several polymeric matrices (polystyrene, sucrose benzoate, polymethylmethacrylate) and are interpreted in terms of three processes : nitrene recombination, hydrogen abstraction from the solvent and reaction of the nitrene with an azido group. Recombination is diffusion-controlled (rate constant ∼109 1. mole–1 sec–1); the same applies to the reaction with an azido group. In contrast, hydrogen abstraction by an aromatic nitrene is a comparatively slow process, corresponding to a life-time (in ethanol) of about 2 × 10–4 sec. This value is corroborated by experiments in soft polystyrene matrices (half-life 10–3sec) and by the rate of intramolecular hydrogen abstraction in the cyclization of 2-nitrenobiphenyl to carbazole (half-life 8 × 10–4 sec). In hard polymeric matrices, hydrogen abstraction is considerably slower (half-lives from 0.1 to 2 sec). Here the rate-determining step is configurational diffusion of CH bonds towards the electron-deficient nitrogen.

Thermodynamics of polymer solutions. Part 4.—Polyisobutylene and n-pentane

Abstract: Activities of n-pentane in solutions with polyisobutylene have been measured at 25°C over the range of concentrations, expressed as the segment fractions φ2 of polymer, from 0.363 to 0.965. These results in conjunction with those of Baker et al.(with which our results agree) yield values of χ that increase from ca. 0.48 at φ2= 0 to ca. 0.90 at φ2= 1.0. Theoretical calculations match the experimental χ at high concentrations of polymer well, but overestimate them somewhat at lower concentrations. The excess volume is calculated to be –1.55 %, in good agreement with experiment. Both the partial molar enthalpy and entropy of dilution are predicted to be negative at high dilutions; the reduced partial molar non-combinatory entropy text-decoration:overlineSR1/Rφ22 is calculated to be ca.–1.0. Thus, the existence of an LCST is predicted; the temperature at which phase separation occurs is not reliably calculable, however, owing to the precision that would be required on the one hand and the limitations of theory on the other. Results for the four systems treated in the present series of papers are compared.

Thermodynamic properties of binary systems containing hexafluorobenzene. Part 4.—Excess Gibbs free energies of the three systems hexafluorobenzene + benzene, touene, and p-xylene

Abstract: The excess Gibbs free energy of the three binary systems hexafluorobenzene + benzene, + toluene, and + p-xylene have been measured in the temperature range 30–70° using a static method. The hexafluorobenzene + benzene system appears to be unique among solutions of non-electrolytes in that it forms a double azeotrope at all the experimental temperatures. All three systems are believed to form equimolar, intermolecular complexes in the liquid, as well as in the solid, states. The experimental free energy data have been analyzed to yield values of the equilibrium constants and enthalpies of formation for the complexing reaction.

Thermodynamics of octamethylcyclotetrasiloxane mixtures

Abstract: Vapour pressures of mixtures of octamethylcyclotetrasiloxane + benzene and octamethylcyclotetrasiloxane + carbon tetrachloride have been measured at 25, 35, 45, 55, and 60°C by a precision static vapour pressure method. For the first system the excess entropy and excess enthalpy are independent of temperature. Excess volumes at 25 and 60°C are also presented; for this system they are small and negative. Results for the second system will require calorimetric studies to finalize conclusions because of uncertainties in the second virial coefficients of carbon tetrachloride. The excess volumes for this system were also negative but larger in magnitude.

Flocculation studies of non-aqueous sterically stabilized dispersions of polymer

Abstract: The incipient instability of non-aqueous sterically stabilized dispersions of polymer particles has been investigated. Flocculation was induced by decreasing the solvency of the dispersion medium for the stabilizing moieties, either by non-solvent addition or by cooling. The influence of such factors as the nature of the dispersion medium, the temperature, the non-solvent, the stabilizing moieties, the anchor polymer, the disperse phase, the particle size and the surface coverage has been examined. Flocculation occurs in dispersion media which are of better solvency for the stabilizing moieties, in free solution, than the corresponding theta-solvents. It is apparently governed by the component of the soluble moieties for which the given flocculating dispersion medium is of least solvency. The results support Fischer' s general approach to steric stabilization.

Longitudinal capillary waves. Part 2.—Experiments

Abstract: It has been shown theoretically that longitudinal waves can easily be studied at an interface with a high elastic modulus. Measurements on mixed monolayers of cholesterol and dimyristoyl lecithin show that the behaviour of longitudinal waves is in accordance with predictions from theory. The elastic modulus at this interface as obtained from the wavelength of longitudinal waves at 200 c/sec, is several times the “equilibrium” modulus obtained from the slope of force-area curves.

Surface pressure and surface potential study of a synthetic phospholipid at the air/water interface

Abstract: Surface pressure and surface potential measurements on monolayers of synthetic 1,2-dipalmitoyl-DL-phosphatidylethanolamine at the air/water interface indicate that the axis of the zwitterion is parallel to the surface. Many two-dimensional arrangements in this plane are energetically possible. Surface phase dissociation constants of the zwitterion functional groups are calculated from surface potential data, and the effect on the monolayer of differing cations in the subphase is described.

Electrical properties of linear polyacetylene

Abstract: Linear polymer of acetylene was prepared in which the oxygen content was as low as 0.7 %. The electrical conductivity and Seebeck voltage but not the energy gap depended on the extent of oxidation of the samples. Adsorbed donor and acceptor gases affected electrical conductivity but not e.s.r. spin assay. It was concluded that the degree of crystallinity in these polymers is of equal importance with chemical purity to achieve high electrical conductivity.

Gas chromatography at finite concentrations. Part 2.—A generalised retention theory

Abstract: The retention theory of gas chromatography is extended to finite concentrations of solute. The detailed treatment takes account of gas compressibility, gas imperfection and variation in velocity of the mobile gas phase due to flux of solute molecules across the interphase boundary. The compressibility-corrected retention volume V∘R of a chromatographic zone of given gas phase concentration c is shown to be related to the gradient (∂q/∂c)p of the partition isotherm by the equation, V∘R=V∘M+V1(1 –ajy0)(∂q/∂c)p. V∘M is the compressibility-corrected gas hold-up; V1 is the volume of solvent component in the stationary phase in gas-liquid chromatography; P is a defined mean column pressure; y0 is the mole fraction of solute in the gas phase at the column outlet, corresponding to concentration c; and a and j represent expressions which, for a gas phase showing only moderate deviation from the perfect gas law, are closely approximated by unity and the conventional James-Martin compressibility factor, respectively. A similar equation holds for gas-solid chromatography. The equation becomes inaccurate at high solute concentrations if gas imperfection is large; estimates of the range of conditions in which the equation is valid are made.

Ultra-violet absorption spectrum of isocyanic acid

Abstract: The absorption spectrum of HNCO between 2000 and 2800 A has been photographed with high resolution and absorption paths of up to 8 m atm. None of the bands are completely sharp, but many show coarse rotational structure. Long progressions of bands are assigned to excitation of an NCO bending vibration. The rotational structure shows that the NCO chain is strongly bent in the excited state, and that either ∠NCO = 119° and rNC+rCO= 2.64 A if the excited state is trans, or ∠NCO = 129° and rNC+rCO= 2.60 A if the excited state is cis. Comparison of the observed band contours with computed asymmetric top band syntheses proves that the bands have type C polarization. The electronic and geometrical structure of HNCO are discussed with the aid of a diagram correlating the molecular orbitals of HABC and AB2 molecules.

Second virial coefficients of argon, krypton and methane and their binary mixtures at low temperatures

Abstract: A differential method is described for measuring the second virial coefficient of a gas from approximately the normal boiling-point of the substance to room temperature, given the value of the virial coefficient at one temperature. The imperfection of the gas is compared directly with that of a reference gas (helium) for which the second virial coefficient is known over the whole temperature range. Measurements have been made on argon, krypton, and methane, and on a sample, approximately equimolar, of each of the three binary mixtures which can be formed from these gases. The results for the pure gases have been used to demonstrate the superiority of the 18-6 form of the Lennard-Jones potential over the 12-6 form in accounting for low-temperature virial coefficients, while the figures obtained for the mixtures have been applied to examine the validity of three combining rules for estimating the intermolecular energy parameters for a pair of unlike molecules from the corresponding parameters for the two pairs of identical molecules.

Volumetric properties of alcohols in dilute aqueous solutions

Abstract: The densities of dilute (mole fraction concentration < 2 × 10–3) aqueous solutions of the four isomeric butanols have been determined at 0.5, 5, 25 and 40°, and solute partial molar volumes and expansibilities have been evaluated. Extrapolations of volume data to infinite dilution are analyzed, and the limiting volumes and expansibilities are interpreted in terms of the effects of alcohols on the structural properties of water. The concentration and temperature dependences of the partial volumes indicate the presence of long-range interactions between hydrated solute molecules.

Reactivity of hydrogen to atomic nitrogen and atomic oxygen

Abstract: The effect of adding 20–50 % H2 to active nitrogen has been studied at 196 and 320° K. At neither temperature is there a primary reaction but when the nitrogen atoms are partially titrated with nitric oxide at 320° K, a catalytic removal of atoms is induced involving the reactions : O + H2= OH + H (5), N + OH = NO + H (8), N + NO = N2+ O (1), O + OH = O2+ H (9), with (5) rate controlling. The consumption of both nitrogen atoms and oxygen atoms was followed photometrically and the results yields a ration k8/k9– 1.4±0.1 at 320° K. The rate constant k5 was (1.2 × 0.1)× 107cm3 mole –1 sec–1at 320° K in separate experiments. The role of H2 as a third body in the recombination reactions of nitrogen and oxygen atoms has been examined at 196° K where reaction (5) is unimportant.

Electron diffraction study of gaseous dimethylamine and trimethylamine

Abstract: The molecular structures of dimethylamine and trimethylamine in the gas phase have been determined accurately by electron diffraction. The C—N bond lengths in the two molecules are equal (1.455 A; reproducibility 0.002 A). This value is 0.010 A less than that obtained by Bartell in methylamine. The C—N—C angles are 111.8 and 110.6°, respectively (reproducibilities 0.6°).

Mössbauer spectra of compounds containing iron(II) in strong-field tetragonal environments

Abstract: Mossbauer spectra of compounds having the general formulae bis(1, 2-cyclohexanedione dioxime) iron (II) XY and phthalocyanineiron(II)(Z)2, where X, Y, Z represent ligands at the axial positions, have been measured at 77°K and in some cases at room temperature. The spectra consist of quadrupole-split doublets, from which two parameters have been obtained : the isomer shift δ and the quadrupole splitting, ΔEQ. These parameters appear to be linearly related to each other within both series of complexes. It is argued that such a relationship exists only when the ligands can be considered to be either pure π-acceptors or pure π-donors.

Character of absorption in far infra-red by polar molecules in liquid state

Abstract: The use of the Fourier spectrometer has been extended down to 2 cm–1 and the results of a study of the absorption and dispersion in the liquid and dissolved states of selected polar molecules (methyl chloroform CH3CCl3; chlorobenzene C6H5Cl; benzonitrile C6H5CN; tolune C6H5CH3) from this wave number to beyond 100 cm–1 are reported. It is concluded that the available representations (Record, Powles) of the decay of the dipole relaxation (Debye) absorption due to the inertia factor are inadequate. The further distinct absorption of polar liquids predicted by Poley has been observed for each of the liquids studied. Systematic evaluation of the nature of these absorptions has shown intense features of considerable breadth located in the wave number range 30 to 80 cm–1. Observed data are :[graphic omitted] This (Poley) absorption is of intensity proportional to the molar concentration and is apparently independent of the solvent on dilution. The wide significance of these data is discussed.

Thermodynamic, ultrasonic, spectroscopic and miscibility studies of water + methyl cyanide solutions

Abstract: The ultrasonic absorption, excess volumes and mutual solubilities of mixtures of methyl cyanide and water are reported together with the near infra-red absorption spectra of 6 m HOD in deuterium oxide containing methyl cyanide. These properties of methyl cyanide and water mixtures are compared with those of tertiary butyl alcohol and water mixtures and discussed in terms of the effects of added methyl cyanide on the structure of water.

Thermodynamics of heterovalent cation exchange reactions in a montmorillonite clay

Abstract: Experimental results are presented for the free energy and enthalpy changes pertaining to the exchange process between four divalent cations (Mg2+, Ca2+, Sr2+ and Ba2+) and two monovalent cations (Na+ and NH+4) in a montmorillonite clay. The free energy changes have been calculated from the exchange isotherms using a procedure which allows the elimination of incomplete attainment of equilibrium and the need of using activity coefficients in the mixed electrolyte solution. Enthalpy changes have been measured calorimetrically and calculated from the temperature coefficient of the equilibrium constant. Enthalpy changes are correlated with the polarizability of the substituting cation. When comparing the divalent cations studied with each other, the replacement of a particular divalent cation by one of larger polarizability gives rise to an entropy loss. A calculation of the electrostatic free energy exchange between mono- and divalent cations is presented. The result is in good agreement with the experimental value of the free energy of exchange observed for the smaller cations, Na+ and Mg2+.

Electrocrystallization of mercury, silver and palladium

Abstract: The kinetics of electrodeposition, on to inert electrodes of vitreous carbon and pyrolitic graphite, of deposits of Ag, Pd and Hg from solutions of their perchlorates, chlorides and nitrates respectively, are described for potentiostatic conditions. For Hg, which requires no lattice incorporation, the slow step is diffusion-controlled growth of three-dimensional nuclei. For Ag, the deposition occurs by two-dimensional growth of plates formed on the top of plates so as to produce three-dimensional nuclei. The slow step appears to be incorporation into the Ag lattice of Ag neutral atoms, the concentration of which is determined by a fast electrochemical pre-equilibrium. This means the measurements can not be interpreted in terms of α, and io for the electrochemical step alone. The constant for the lattice incorporation step at the reversible potential is about 5 × 10–5 moles cm–3 sec–1 for a 1.5 × 10–2 M solution of silver nitrate. For Pd, deposition occurs by three-dimensional nucleation and growth. In contrast to silver, the slow step is the electrochemical reaction preceding crystal growth. The rate constant at the reversible potential is about 3.3 × 10–9 moles cm–2 sec–1, and α= 0.6 for a 5.12 × 10–2 M palladous chloride solution.

Thermodynamics of polymer solutions. Part 3.—Polyisobutylene and cyclohexane

Abstract: Activities and excess volumes for concentrated mixtures of polyisobutylene and cyclohexane have been determined at 25°C. The former yield χ≃ 0.5; variations of χ with concentration, expressed as segment fractions, are small. The volume change on mixing is negative, amounting to ca. –0.14% of the total volume at φ2= 0.5. These results in conjunction with the previously determined enthalpy of mixing are rationally interrelated by theory; arbitrary assignment of X12≅ 1.4 cal cm–3 suffices. This value of X12 denotes a very small difference for the interactions between neighbour pairs. The dependence of χ on concentration is overestimated by theory, but the magnitude of the large entropic contribution to χ is correctly predicted.

Contact angle between black film and bulk liquid

Abstract: The contact angles between black film and bulk liquid from aqueous solutions of NaLS are measured at different NaCl concentrations both by topographical and expansion methods. The contact angles increase about 10 times during the transition from first to second black films. The rate of expansion of the black films is studied; the critical thickness of the films by which black spots are formed coincide with the critical thickness of rupture. The data for the contact angles and for the kinetics of expansion of the black films are interpreted with the aid of the concept for the film tension.